Positive flow control systems using open-centered control valves are known. In such systems, a fluid pump provides a flow of fluid to various systems on the vehicle. Fluid flow is continuous at variable rates, sequentially passing through two or more open-centered valves such that operation of various actuators controlled by each of the valves is prioritized. For example, a vehicle having a loader implement may have an open-centered hydraulic system that prioritizes operation of a tilt actuator over a lift actuator by placing the control valve for the tilt actuator upstream of the valve for the lift actuator.
One example of such a hydraulic system can be found in U.S. Pat. No. 5,873,244, issued on Feb. 23, 1999, to Cobo et al. (the '244 patent), the contents of which are incorporated herein in their entirety by reference. The '244 patent discloses a positive flow control system using open-centered control valves connected in series. The system described in the '244 patent uses an orifice placed downstream of the series of valves to regulate flow of pumped fluid passing through each valve when all valves are in a neutral position. One disadvantage of the system disclosed in the '244 patent is that accurate control of fluid flow through the pump when all control valves are in their neutral position is not easily controllable. Another disadvantage is that, typically, the open-centered control valves are calibrated at high engine speeds with hot hydraulic fluid. This arrangement yields inconsistent command dead band when operating at conditions different than the calibration conditions. Moreover, pressure in a typical open-centered system is higher than required when the speed of the engine is high and the temperature of the lubrication fluid is low, and lower than required when the speed of the engine is low and the temperature of the lubrication fluid is high. Under such conditions, the vehicle may experience inadequate lubrication when the pressure is low or waste engine power when the pressure is high.